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The
change in energy can be plotted against the progress of a reaction, as the reactants
turn into products.
Going
from reactants to the top of the curve, you are going up the energy scale.
Energy (heat) is being put in
to break bonds in the reactants.
At the
top of the curve, the bonds in the reactants have been broken.
The amount of energy put in to break these bonds is called the activation
energy.
The
activation energy is the minimum amount of energy needed for the reaction to
occur.
A catalyst may work by
lowering the activation energy for a reaction.
Going
from the top of the curve to the products, you are going down the energy scale.
Energy (heat) is given out as
bonds form in the products.
The
reactants are higher up the energy scale than are the products.
The amount of energy (heat) you need to put in (the activation energy) is less
than the amount of energy (heat) you get out.
This is a typical exothermic reaction.
The
difference in energy levels between the reactants and the products is given the
symbol ∆
H (the
enthalpy change or the heat of reaction).
This is the amount of heat given out (or taken in) during the reaction.
For an exothermic reaction,
∆
H is
negative.
For an endothermic reaction,
∆
H is
positive.
[ Exothermic and endothermic changes ] [ Energy level diagrams ] [ Activation energy ] [ Rates of reaction ] [ Factors affecting rate of reaction ]
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